binfmt_misc problems in ubuntu18.04 under docker

Question

Trying to get a cross chroot running under ubuntu:18.04 in a docker container.

While trying to install binfmt-support (dependency of qemu-user-static) this happens:

~ docker run -it ubuntu:18.04 /bin/bash
root@df5a34a47fe2:/# cd
root@df5a34a47fe2:~# apt-get update
...
root@df5a34a47fe2:~# apt-get install qemu-user-static
...
Setting up qemu-user-static (1:2.11+dfsg-1ubuntu7.9) ...
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
mount: /proc/sys/fs/binfmt_misc: permission denied.
update-binfmts: warning: Couldn't mount the binfmt_misc filesystem on /proc/sys/fs/binfmt_misc.
Setting up binfmt-support (2.1.8-2) ...
invoke-rc.d: could not determine current runlevel
invoke-rc.d: policy-rc.d denied execution of start.
root@df5a34a47fe2:~# echo $?
0

Does anybody know what's happening here, if it's relevant and how to fix it?

Answer 1

binfmt is trying to register qemu as a binary loader so you don't need to invoke qemu everytime you need to run a binary with it. To do that it requires privileges that the container doesn't have.

Ignore those warnings because it doesn't make sense to register binfmt on the host while the image is being built.